The PBP website:
We might be led to restrict the number of registrations in order to ensure the safety and quality of this event
An unofficial PBP does increase the difficulty the organisers have in achieving their goals of a safe/quality event. In this thread, a quick review of the adjectives describing those who "ride anyway" includes immoral (or at least impolite), a failure, an arse, a freeloader, a twat, a Kudos seeker, a bit sad, a thief*, an anarchist, dickish, swampists... and I'm not sure but maybe even shittists (?).
On the positive side, at least one person riding an event they hadn't officially entered was a charitable fundraiser.
The consensus here - even among those offering riders the benefit of the doubt - doesn't sound hugely positive. If anyone is searching Google for 'unofficial PBP' I hope they find this resource informative and that, should someone choose to go ahead anyway, they'll have accepted an appropriate adjective for the duration.
*debatable and debated